This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. We wish to use fluorescent recovery after photobleaching (FRAP) to measure lateral diffusion rates of fluorescently labeled receptors CD4 and CCR5 on cell membranes when exposed to ligands such as gp120 protein or gold nanoparticle-tagged antibodies to the receptors. We have transiently transfected cells expressing these proteins and we would expose them to the ligands shortly before imaging them. We wish to measure the change in diffusion dynamics of these receptors when bound to these ligands.